Loss current diminisher for compact neutron source



Dec. 19, 1961 c. H. GOLDIE 3,014,132 LOSS CURRENT DIMINISHER FOR COMPACT NEUTRON SOURCE Filed Jan. 2, 1959 lllllllllllllll'kg This invention relates to compact neutron sources of the type described in co-pending application Serial No. 515,435, which issued on October 6, 1959 as US. Patent No. 2,907,884 assigned to the assignee of the present invention. As described more fully therein, said application relates to a compact neutron source of generally tubular configuration wherein an axially located positive ion source provides necessary positive ions which are then accelerated radially outward onto a target which is supported upon the inside surface of the tubular container enclosing the neutron source. One of the problems associated with such a neutron source is the fact that power is wasted in the form of secondary electrons emitted near the target which are then accelerated towards the positive ion source. These secondary electrons are produced by positive-ion bombardment at a rate of from two to twenty secondary electrons for each positive ion and the entire ion-acceleration voltage is available to impart energy to the secondary electrons. The problem is thus radically different from secondary emission problems in conventional radio tubes wherein secondary electrons are seldom emitted at more than a one-to-one ratio by electron bombardment and where the main volt age drop between cathode and anode tends to decelerate rather than accelerate the secondary electrons. In accordance with the teachings of said application, a suppressor grid may be employed in order to reduce this secondary electron current. Such a suppressor grid presents problems of its own, including the fact that it is not fully transparent and also the fact that secondary and field emission may result from the grid. In accordance with the invention, secondary electron current from the target area is reduced by producing a reversed field in the vicinity of the target by means of conductive members positioned out of the path of the positive-ion beam. In a preferred embodiment of the invention two well-rounded rings of conductive material are provided, one on eitherside of the target but insulated therefrom. An appropriate negative bias is then put on the two suppressor rings with respect to the target. In this Way, electron suppression is accomplished without interposing anything in the path of the positive ion beam. Because the rings project inwardly from the tubular container, it is possible that the available voltage for ion acceleration may be reduced. If this is the case, one may use smaller rings, which then act as a Faraday cage or electric shield, and supplement the action of the Faraday cage by a grid. The Faraday cage then serves to reduce the field at the grid so as to minimize field emission at that point. Of course, this construction has the disadvantage of being somewhat less transparent to the positive ion beam than the construction first described.

The invention may best be understood from the following detailed description thereof having reference to the accompanying drawing in which FIG. 1 is a view illustrating one embodiment of the invention; and

FIG. 2 is a view illustrating a second embodiment of the invention.

Referring to the drawings and first to FIG. 1 thereof, the positive ion source 1 is supported along the axis of a tube 2. Positive ions are created within the positive ion source in a manner not necessary to explain herein in any detail, and are then accelerated radially outwardly towards a target 3 which is supported upon the inner surface of the tubular member. The positive ions are accelerated by means of an electric field which is produced in general by raising the positive ion source to high voltage while the tubular container is grounded. The construction of such a device is described in great detail in the aforementioned co-pending application. Insulating supports 4 are provided adjacent the target and upon these a pair of rings 5 are supported. These rings are maintained at a slightly negative potential with respect to the tubular casing by means of an appropriate voltage source 6 which is connected to the upper ring by means of an appropriate lead 7. The upper ring in turn is connected to the lower ring by means of a post or two 8. In this way the effect of a suppressor grid such as that described in the aforementioned co -pending application is obtained without intercepting the positive ion beam, without the emission of secondary electrons from the grid and Without the danger of field emission from the grid.

As appears from the illustration in FIG. 1, the rings 5 produce a region in space which is at the potential of the rings and which protrudes inwardly from the tubular casing and therefore reduces the voltage that may be impressed or maintained on the positive ion source. If this limitation results in a requirement that the rings be of very small dimensions, the effect of a conventional grid 9 may be supplemented by making use of the fact that the rings 5' form a Faraday cage, or shielded region, therebetween. Such an arrangement is shown in FIG. 2. The use of such rings in accordance with the invention in combination with a suppressor grid is advantageous since it reduces the electric field at the grid and thereby minimizes the danger of field emission at that point. The construction shown in FIG. 2 is identical to that shown in FIG. 1 except that the conductive posts 8 of FIG. 1 are replaced by a conventional grid 9 which maintains electrical contact between the two suppressor rings 5.

Having thus described the principles of the invention together with several illustrative embodiments thereof, it is to be understood that although specific terms are employed, they are used in a generic and descriptive sense and not for purposes of limitation, the scope of the invention being set forth in the following claims.

I claim:

1. A compact neutron source comprising'in combination a source of positive ions, a target adapted to produce neutrons upon bombardment by said positive ions and supported circumferentially about and spaced from said source of positive ions, means for maintaining said source of positive ions at a positive potential with respect to said target suf'ficient to accelerate ions from said ion source onto said target, conductive members supported in the vicinity of said target but insulated therefrom and out of the path of but adjacent to the positive-ion beam, and means for maintaining said conductive members at a negative potential with respect to said target, whereby a reversed field is produced in the vicinity of said target.

2. A compact neutron source comprising in combination a source of positive ions, a target adapted to produce neutrons upon bombardment by said positive ions, and supported circumferentially about and spaced from said source of positive ions, means for maintaining said source of positive ions at a positive potential with respect to said target suflicient to accelerate ions from said ion source onto said target, at least two conductive rings supported in the vicinity of said target but insulated therefrom and flanking the path of the positive-ion beam, and means for maintaining said conductive rings at a negative potential with respect to said target, whereby a reversed field is produced in the vicinity of said target.

3. A compact neutron source comprising in combination a source of positive ions, a target adapted to produce neutrons upon bombardment by said positive ions and supported circumfe-rentially about and spaced from said source of positive ions, means for maintaining said source of positive ions at a positive potential with respect to said target sufiicient to accelerate ions from said ion source onto said target, a suppressor grid supported in the vicinity of said target but insulated therefrom, conductive members flanking the path of said positive-ion beam and arranged in electrical connection with said suppressor grid so as to provide a Faraday cage efieot at said suppressor grid, and means for maintaining said suppressor grid and said conductive members at a negative 4 potential with respect to said target, whereby a reversed field is produced in the vicinity of said target.

References Cited in the file of this patent UNITED STATES PATENTS Johnson Sept. 16, 1958 

